Security is becoming increasingly important as the internet and electronic devices become more pervasive. For example, computers and even mobile telephones are equipped with biometrics to prevent access by unauthorized users.
Encryption is also used to prevent unauthorized access to devices and information. For example, data can be encrypted before being transmitted on the internet. Other techniques, such as security tokens, are also employed to limit access to devices.
In addition, many electronic systems require a unique digital identifier for authentication, key derivation and other purposes. These electronic systems are often manufactured using traditional manufacturing processes. Creating a unique digital identifier in this environment is often difficult and time consuming. Furthermore, to be effective, the unique digital identifier should be extremely different or nearly impossible to determine and copy.
One method of creating this unique digital identifier is through the use of waveguides. FIG. 1 shows a cross section of a printed circuit board 10 with a conventional planar waveguide 20. The printed circuit board 10 includes one or more light sources 11. These light sources 11 emit light that enters the waveguide 20 by means of angle mirror 26 cut into the waveguide 20. The light initially appears in both the inner core 21 and the outer cladding 22, but an absorptive layer of material 25 absorbs the light in the outer cladding 22. The printed circuit board 10 also includes an image sensor 12, such as a CCD image sensor. Light in the inner core 21 is not coupled to the image sensor 12, but inhomogeneities 27 in the inner core 21 scatter light into the outer cladding 22 where some fraction of this light is received by the image sensor 12. Thus, some portion of the light emitted from the light sources 11 reaches the image sensor 12. The light pattern created on the image sensor 12 is then converted to a digital value. Slight differences in the structure of the waveguide 20 affect the resulting light pattern, causing unique patterns to be reflected onto the image sensor 12. Thus, the light pattern represents the unique identifier.
As mentioned above, these waveguides 20 are traditionally constructed using an inner core 21 surrounded by an outer cladding 22. The outer cladding 22 is then covered by a reflective silver layer 24. The inner core 21 may have a higher refractive index (n) than the outer cladding 22. For example, the inner core 21 may have a refractive index of 1.59, while the outer cladding has a refractive index of 1.49. Light is reflected at the boundary between the inner core 21 and the outer cladding 22 or at the boundary between the outer cladding 22 and the silver layer 24.
As shown in FIG. 1, the incident angle of the light determines at which boundary the light is reflected. Higher incident angle light is reflected at the boundary between the inner core 21 and the outer cladding 22, while lower incident angle light is reflected at the silver layer 24. For example, using the refractive indices described above, light with an incident angle of 70° to 90° will remain trapped in the inner core 21. Light with a lower incident angle, such as 60° to 70°, are contained within both the inner core 21 and the outer cladding 22. Further, at incident angles less than roughly 60°, the light will exit the outer cladding 22 and may be reflected by the silver layer 24.
FIG. 2 shows a top view of the waveguide 20 of FIG. 1. Disposed under the waveguide 20 are a light source 11 and an image sensor 12. Light is emitted from the light source 11 and traverses the waveguide 10 to the image sensor 12. FIG. 2 also shows an intrusive probe 13 that has been inserted into the waveguide 10. If the probe 13 is not inserted into the direct path between the light source 11 and the image sensor 12, its affect on the reflected light pattern received by the image sensor 12 may be minimal. For example, there may be some small amount of light 15 reflected off the probe 13 that may affect the reflected pattern; however, most of the light in the waveguide 20 that is destined for the image sensor 12 is unaffected by the probe 13. If the probe 13 is not inserted in the direct light path, the shadow 14 cast by the probe 13 may have no affect on the reflected light pattern received by the image sensor 12.
However, ideally, the light pattern should be significantly affected by the insertion of an intrusive probe 13, regardless of the location of that insertion. Therefore, it would be beneficial if there were a waveguide where the reflected light pattern is more significantly affected by the insertion of a probe. Furthermore, it would be advantageous if this significant change in the reflected light pattern occurred regardless of the location of the insertion.